Temporal variation of mineralogy and petrology in cognate gabbroic enclaves at Arenal volcano, Costa Rica

Abstract

Gabbroic enclaves ejected during the current eruption phase (A-1) and during the latest prehistoric eruption phase (A-2) of Arenal Volcano show systematic variations in texture, mineralogy and composition as a function of host rock chemistry and timing of eruption. The most differentiated enclaves occur in the more differentiated A-2 lavas. Enclaves in the A-1 volcanics are consistently less evolved. Within the current A-1 eruption, the most mafic enclaves are amphibole-bearing rocks that were erupted during the first 2–3 years of activity (1968–1970). These enclaves occur in the most differentiated A-1 volcanics and are not in equilibrium with their host rocks. They crystallized from a hydrous melt that was slightly more mafic than anything erupted during the current cycle. We interpret the enclaves as sidewall crystallization products of a melt, possibly a high-alumina basalt, that was immediately parental to the A-1 lavas. Enclaves that occur in A-1 rocks erupted after 1970 and all of the A-2 enclaves are amphibole-free and less mafic than the early A-1 enclaves. Their chemistry suggests that they formed during the early to intermediate crystallization of their host lavas. None of the enclaves contain minerals that might have equilibrated with a primary, mantle-derived melt. Geothermometry is consistent with geochemistry, with amphibole-bearing A-1 enclaves yielding the highest pyroxene temperatures (ave. 1090° C) and A-2 enclaves the lowest (ave. 1030° C). Geobarometry suggests mid- to upper crustal depths for the crystallization of all enclaves. The enclaves are cognate and reflect pre-eruptive crystallization of Arenal magmas. They record evolution from a hydrous, basaltic magma to the drier basaltic andesites that characterize the current eruption. Volatiles appear to have been lost due to depressurization during the slow ascent of the magmas through the upper levels of the crust following the initial explosive eruption. Volatile loss and depressurization resulted in the destabilization and the progressive resorption of amphibole. The A-2 lavas may represent the long-term fractionation products of basaltic andesite magmas similar in composition to the A-1 lavas. Anorthitic plagioclase, commonly thought of as a phase stabilized by high Ca/Na and high water pressure, continued to crystallize in a system with relatively low Ca/Na and which had dehydrated and/or depressurized to the point at which amphibole was no longer stable. This suggests that compositional characteristics other than high Ca/Na or high water content may have stabilized the anorthite in the basaltic and basaltic andesite melts at Arenal. We speculate that the high-alumina content of the Arenal magmas may be the stabilizing factor.